The quantum phenomenon of tunneling enables novel charge-based devices with ultra-low power consumption, and is key to the emerging field of spintronics.
Tunnel devices typically require mating dissimilar materials and maintaining monolayer level control of thickness, raising issues that severely complicate fabrication and compromise performance. The recent discoveries of intrinsically 2-dimensional materials such as graphene and h-BN have created new perspectives on tunnel barriers. Their strong in-plane bonding promotes self-healing of pinholes and a well-defined layer thickness, important because the tunnel current depends exponentially upon the barrier thickness.
There has been keen interest in utilizing graphene, a two-dimensional (2D) honeycomb lattice of carbon atoms, as a high mobility transport channel. Its linear band dispersion, ambipolar conduction, and remarkable in-plane electronic transport properties have stimulated development of RF transistors and wafer-scale fabrication of graphene circuits. Graphene also exhibits exceptional in-plane spin transport characteristics, including long spin diffusion lengths due to its low spin-orbit interaction, which has stimulated ideas for novel spin devices.
The highest values for spin diffusion lengths and spin lifetimes have been measured using mechanically exfoliated graphene, which, although it possesses extraordinary electrical properties, is not amenable for device scalability, as devices must be fabricated on individual, randomly placed and sized flakes. Moreover, spin injection into graphene from a ferromagnetic metal contact typically requires the use of an oxide tunnel barrier such as Al2O3 or MgO to accommodate the large conductivity mismatch. These materials do not wet the graphene surface, making it very difficult to control the thickness and uniformity of the tunnel barrier.
In addition, the mobility of graphene is significantly degraded by coupling to phonons or charged impurities/defects in an adjacent oxide. Consequently, significant effort has focused on exploiting other carbon thin films and 2D materials such as h-BN or MoS2 as a substrate, gate dielectric, or tunnel barrier for graphene devices. This improves operating characteristics, but significantly complicates the fabrication, and often relies upon sequential mechanical exfoliation to produce a few device structures.
Although single layer graphene itself has been shown to function as a tunnel barrier in a heterostructure, it does not effectively serve as a tunnel barrier on another layer of graphene because there is electrical interaction between the two layers, regardless of the stacking orientation, except in a large magnetic field.
One can markedly alter graphene's physical properties with chemical functionalization by fluorination or hydrogenation. Fluorinated graphene is an excellent in-plane insulator, and no electrical communication is observed between adjacent layers of fluorographene and graphene, allowing for its use as a tunnel barrier in an all-graphene tunnel-transport homoepitaxial structure.
Only two other methods have been devised for making tunnel barriers on graphene. First, a method of high-energy electron-beam lithographic decomposition of vaporized carbon can produce amorphous carbon layers on the surface of the graphene channel and can act as a tunnel barrier. Although this method produces tunnel barriers, the high-energy electron beam adds charged impurities to the substrate, affecting the transport properties of the graphene channel, and it can induce physical damage to the graphene by driving off individual carbon atoms from the lattice. A second alternative method involves the chemical vapor deposition growth of thin layer hexagonal-BN, which is then transferred to the graphene transfer. However, this process does not produce exceptional results and is not homoepitaxial, requiring the growth and transfer of two completely different materials with vastly different growth mechanisms and properties. Thus, it is also unsuitable for industrial scaling.